debian/rules: Don't muck with m4/

There's no need to install or remove m4/ anymore, a placeholder is
installed by upstream now.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
debian/gbp.conf: Use the appropriate upstream-tag

Upstream releases are tagged libmongo-client-$VERSION, teach gbp about
that.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>